Vulcanizable rubber composition with unsaturated and metal compounds and organic peroxides

ABSTRACT

This invention relates to a vulcanizable rubber composition obtained by compounding, into an ethylenically unsaturated nitrile-conjugated diene copolymer rubber, a zinc compound, an α, β-ethylenically unsaturated carboxylic acid, a compound of a 2A Group metal of the periodic table and an organic peroxide. The vulcanizable rubber composition of this invention can provide a vulcanized rubber having excellent heat resistance and oil resistance and moreover possessing high tensile stress or stably exhibiting high tensile strength without being affected by kneading conditions, and is hence suitable for use as a rubber material for toothed belts and the like.

TECHNICAL FIELD

The present invention relates to a vulcanizable rubber compositioncomprising an ethylenically unsaturated nitrile-conjugated dienecopolymer rubber, and more particularly to a vulcanizable rubbercomposition which can provide a vulcanized rubber having excellent heatresistance and oil resistance and moreover stably exhibiting hightensile strength without being affected by kneading conditions.

The vulcanizable rubber composition according to the present inventionis suitable for use in, particularly, fields of which high tensilestress or tensile strength is required, for example, as a rubbermaterial for toothed belts, V-belts, packings, hoses, rubber rolls andthe like.

BACKGROUND ART

It has heretofore been known that when the zinc salt of methacrylicacid, such as zinc dimethacrylate or basic zinc methacrylate, iscompounded into an ethylenically unsaturated nitrile-conjugated dienecopolymer rubber, and the rubber is vulcanized with an organic peroxide,a vulcanized rubber excellent in various physical properties such astensile strength can be obtained. The zinc salt of methacrylic acid maybe compounded either in the form of the salt itself prepared in advanceor by compounding methacrylic acid and a zinc compound (zinc oxide, zinccarbonate, zinc hydroxide or the like) into the copolymer rubber andreacting them in the rubber, thereby forming the salt.

For example, Japanese Patent Application Laid-Open No. 306441/1989 hasdisclosed that a vulcanizable rubber composition obtained by compoundingmethacrylic acid, a zinc compound and an organic peroxide into anethylenically unsaturated nitrile-conjugated diene copolymer rubberprovides a vulcanized rubber having high tensile strength. JapanesePatent Application Laid-Open No. 306443/1989 has disclosed that avulcanizable rubber composition obtained by compounding methacrylicacid, a zinc compound and an organic peroxide into an ethylenicallyunsaturated nitrile-conjugated diene copolymer rubber containingconjugated diene units in an amount of at most 30 wt. % provides avulcanized rubber exhibiting extremely high tensile strength comparedwith the conventional rubbers.

However, the conventional vulcanizable rubber compositions obtained byblending methacrylic acid, a zinc compound and an organic peroxide withan ethylenically unsaturated nitrile-conjugated diene copolymer rubberhas involved a problem that they cannot provide vulcanized rubbershaving sufficiently high tensile stress.

When a rubber material is used in a belt, hose or the like, it must havehigh tensile stress. In particular, a rubber material having hightensile stress together with properties such as excellent heatresistance, oil resistance, tensile strength and processability isrequired for the application to various toothed belts driven under ahigh load. A toothed belt is generally formed from a surface coatinglayer composed of canvas or the like, a rubber layer for forming andholding a toothed part (tooth form), and a reinforcing cord (core)embedded in the rubber layer, and is used as a power transmission means.The toothed belt suffers high stress at the root of each tooth whentransmitting the power in mesh with a pulley. In order to withstand thishigh stress, the tensile stress of the rubber layer which forms andholds the toothed part must be made high.

In general, as an example of a method for enhancing the tensile stressof rubber, may be mentioned a method in which the compounding proportionof a reinforcing material for the rubber, such as carbon black or silicais increased to improve the hardness of the resulting vulcanized rubber.In this case, however, the viscosity of an unvulcanized rubber becomestoo high, resulting in deteriorated processability.

Examples of other methods for enhancing the tensile stress of rubberinclude a method in which the amount of a vulcanizing agent isincreased, and a method in which a crosslinking aid such as triallylisocyanulate or trimethylolpropane triacrylate is added. However, thesemethods involve problems that the resulting vulcanized rubber isdeteriorated in elongation and that a disadvantage is also encounteredfrom the viewpoint of cost.

In addition, the vulcanizable rubber composition obtained by compoundingmethacrylic acid, a zinc compound and an organic peroxide into anethylenically unsaturated nitrile-conjugated diene copolymer rubber haveinvolved a problem that strength properties such as tensile strengthvary according to preparation conditions such as a kneading temperature.In a vulcanizable rubber composition, it is desirable that its strengthproperties should not be affected by variations in temperatureconditions upon the kneading. Further, the kneading of the vulcanizablerubber composition at a too high temperature tends to cause thedeterioration of the copolymer rubber and the reduction of physicalproperties of the resulting vulcanized rubber due to polymerization ofthe zinc salt of methacrylic acid and the like, and applies a heavy loadto a kneading machine. It is therefore desirable that a vulcanizablerubber composition should stably provide a vulcanized rubber having highstrength properties even when kneaded at a comparatively lowtemperature.

Accordingly, it has been an important technical problem to be solved inthe vulcanizable rubber compositions containing an ethylenicallyunsaturated nitrile-conjugated diene copolymer rubber as described aboveto highly improve the tensile stress of the resulting vulcanized rubberor to provide a vulcanized rubber stably exhibiting high tensilestrength even when preparation conditions such as kneading temperaturevary. However, there has heretofore not been found under thecircumstances any useful means for solving such a technical problem.

It has heretofore been known to cause various additives to coexist uponthe reaction of methacrylic acid and a zinc compound in a rubber to formzinc methacrylate therein.

For example, Japanese Patent Application Laid-Open No. 37459/1982 hasdisclosed a method in which calcium oxide or active aluminum oxide isadded as a dehydrating agent upon the reaction of methacrylic acid andzinc oxide in a rubber with a small amount of a natural rubber or apolyisoprene rubber blended into a polybutadiene rubber, therebyadsorbing and dehydrating water formed by the reaction. According tothis method, a vulcanized rubber having high impact resilience and fitfor golf balls can be obtained.

Japanese Patent Application Laid-Open No. 19344/1983 has disclosed thefact that when calcium hydroxide is caused to exist upon the reaction ofmethacrylic acid and zinc oxide in a polybutadiene rubber, the formationof basic zinc methacrylate is prevented, thereby obtaining a vulcanizedrubber having high impact resilience.

Japanese Patent Application Laid-Open No. 101131/1983 has disclosed thefact that when the calcium salt of methacrylic acid is caused to existupon the reaction of methacrylic acid and zinc oxide in a polybutadienerubber, zinc dimethacrylate is formed, thereby obtaining a vulcanizedrubber having high impact resilience.

However, all of these prior art documents relate to golf balls makinguse of a rubber composed principally of a polybutadiene rubber and haveas their object the improvement of impact resilience.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide an ethylenicallyunsaturated nitrile-conjugated diene copolymer rubber composition whichcan provide a vulcanized rubber stably exhibiting high tensile strengthwithout being affected by preparation conditions such as kneadingtemperature, and desirably even upon kneading at a comparatively lowtemperature.

It is another object of the present invention to provide vulcanizedrubber moldings such as a toothed belt improved in tensile strength.

The present inventors have carried out an extensive investigation. As aresult, it has been found that when an α,β-ethylenically unsaturatedcarboxylic acid and a zinc compound are compounded into an ethylenicallyunsaturated nitrile-conjugated diene copolymer rubber to form the zincsalt of the α,β-ethylenically unsaturated carboxylic acid, and thecopolymer rubber is vulcanized with an organic peroxide, therebyproducing a vulcanized rubber, the above objects can be achieved bycompounding a compound of an alkaline earth metal into an unvulcanizedrubber composition.

Among compounds of 2A Group metals of the periodic table, in particular,the use of a magnesium compound has been found to highly improve tensilestress. (Referential Example). The use of a compound of an alkalineearth metal such as calcium can achieve high tensile strength even whenpreparation conditions such as kneading temperature vary, or kneading isperformed at a comparatively low temperature. Accordingly, when the kindof the compound of a 2A Group metal of the periodic table is selected asnecessary for the end application intended, or two or more compounds aresuitably combined, a vulcanizable rubber composition and a vulcanizedrubber having the desired properties can be obtained. The presentinvention has been led to completion on the basis of these finding.

According to the present invention, there is thus provided avulcanizable rubber composition obtained by compounding, per 100 partsby weight of an ethylenically unsaturated nitrile-conjugated dienecopolymer rubber, the following components:

(a) 5-80 parts by weight of a zinc compound;

(b) 5-100 parts by weight of an α,β-ethylenically unsaturated carboxylicacid;

(c) 0.5-30 parts by weight of a compound of an alkaline earth metal; and

(d) 0.2-10 parts by weight of an organic peroxide.

According to the present invention, there is also provided a vulcanizedrubber molding obtained by molding the vulcanizable rubber compositiondescribed above in the desired form and then vulcanizing the thus-moldedrubber composition.

According to the present invention, there is further provided a toothedbelt obtained by using the vulcanizable rubber composition describedabove to mold a tooth back and a toothed part of the toothed belt andthen vulcanizing the thus-molded rubber composition.

The present invention will hereinafter be described in detail.

As examples of the ethylenically unsaturated nitrile-conjugated dienecopolymer rubber useful in the practice of the present invention, may bementioned copolymer rubbers of an ethylenically unsaturated nitrile suchas acrylonitrile, methacrylonitrile or α-chloro-acrylonitrile with aconjugated diene such as 1,3-butadiene, isoprene, 1,3-pentadiene or2,3-dimethyl-1,3-butadiene; hydrogenated copolymer rubbers obtained byhydrogenating the conjugated diene units of these copolymer rubbers;polymonomeric copolymer rubbers of two monomers of an ethylenicallyunsaturated nitrile and a conjugated diene with at least one monomercopolymerizable therewith, for example, a vinyl aromatic compound, analkyl ester of an ethylenically unsaturated carboxylic acid, analkoxyalkyl ester of an ethylenically unsaturated carboxylic acid, afluoroalkyl ester of an ethylenically unsaturated carboxylic acid, acyano-substituted alkyl ester of (meth)acrylic acid or the like; andhydrogenated copolymer rubbers obtained by hydrogenating the conjugateddiene units of these polymonomeric copolymer rubbers.

Specific examples of the ethylenically unsaturated nitrile-conjugateddiene copolymer rubber include acrylonitrile-butadiene copolymer rubber(NBR), acrylonitrile-butadiene-isoprene copolymer rubber (NBIR),acrylonitrile-isoprene copolymer rubber (NIR),acrylonitrile-butadiene-acrylate copolymer rubber,acrylonitrile-butadiene-acrylic acid copolymer rubber,acrylonitrile-butadiene-acrylate-methacrylic acid copolymer rubber, andhydrogenated copolymer rubbers obtained by hydrogenating the abovecopolymer rubbers, such as a hydrogenated product ofacrylonitrile-butadiene copolymer rubber (HNBR). These copolymer rubbersmay be used either singly or in any combination thereof.

These copolymer rubbers contain ethylenically unsaturated nitrile unitsin a proportion of generally 10-60 wt. %, preferably 20-50 wt. % intheir polymer chains. If the content of the ethylenically unsaturatednitrile units is lower than 10 wt. %, the resulting vulcanized rubber isinsufficient in oil resistance. On the other hand, any content exceeding60 wt. % results in a vulcanized rubber deteriorated in resiliency. Itis hence not preferable to use any copolymer rubber containing theethylenically unsaturated nitrile units in a proportion outside theabove range.

As examples of the zinc compound, may be mentioned zinc oxide, zinccarbonate and zinc hydroxide.

As examples of the α,β-ethylenically unsaturated carboxylic acid, may bementioned unsaturated monocarboxylic acids such as acrylic acid,methacrylic acid, crotonic acid and 3-butenic acid; unsaturateddicarboxylic acids such as maleic acid, fumaric acid and itaconic acid;monoesters of unsaturated dicarboxylic acids, such as monomethylmaleate, monomethyl fumarate and monomethyl itaconate; unsaturatedpolycarboxylic acids other than the above; and esters of the unsaturatedpolycarboxylic acids, in which at least one free carboxyl group remainsintact. Of these, methacrylic acid is particularly preferred from theviewpoint of physical properties and easy availability.

When the zinc compound and the α,β-ethylenically unsaturated carboxylicacid are added to the ethylenically unsaturated nitrile-conjugated dienecopolymer rubber, and the resultant mixture is kneaded, they are reactedin situ in the copolymer rubber to form the zinc salt of theα,β-ethylenically unsaturated carboxylic acid. In this case, it ispreferable from the viewpoint of improvement in strength properties suchas tensile strength of the resulting vulcanized rubber to use, as thezinc compound, that from which coarse particles having a particlediameter of at least 20 μm have been removed by classification inadvance, thereby reducing the coarse particle content thereof to at most5 wt. %.

The compounding proportion of the zinc compound is generally 5-80 partsby weight, preferably 10-60 parts by weight per 100 parts by weight ofthe ethylenically unsaturated nitrile-conjugated diene copolymer rubber.The compounding proportion of the α,β-ethylenically unsaturatedcarboxylic acid is generally 5-100 parts by weight, preferably 20-60parts by weight per 100 parts by weight of the ethylenically unsaturatednitrile-conjugated diene copolymer rubber. A molar ratio of theα,β-ethylenically unsaturated carboxylic acid to the zinc compoundcalculated on the basis of the molecular weight of the α,β-ethylenicallyunsaturated carboxylic acid and the formula weight of the zinc compoundis generally within a range of from 1:0.5 to 1:3.2, preferably from1:0.5 to 1:2.5.

In the present invention, the compounding of a compound of an alkalineearth metal permits the obtainment of a vulcanizable rubber compositionwhich can stably provide a high-strength vulcanized rubber withoutchanging strength properties according to its preparation conditionssuch as kneading temperature.

As examples of the compound of the alkaline earth metal, may bementioned the oxides, hydroxides, peroxides, carbonates, carbonateoxides, carbonate hydroxides, sulfates, nitrates, acetates, oxalates,phosphinates, phosphonates, phosphates, hydrogenphosphates and ammoniumphosphates of calcium, strontium, barium and the like.

As specific examples of the compound of the alkaline earth metal, may bementioned. calcium compounds such as calcium oxide, calcium peroxide,calcium hydroxide, calcium sulfate, calcium nitrate, calcium, carbonate,calcium acetate, calcium oxalate, calcium phosphinate, calciumphosphonate and calcium phosphate; strontium compound such as strontiumoxide, strontium peroxide, strontium hydroxide, strontium sulfate,strontium nitrate, strontium acetate, strontium carbonate, strontiumoxalate and strontium phosphate; barium oxide, barium peroxide, bariumhydroxide, barium sulfate, barium nitrate, barium carbonate, bariumoxalate, barium phosphinate, barium phosphonate and barium phosphate;and mixtures of two or more these compounds.

Of these metal compounds, the oxides and hydroxides of the alkalineearth metals are preferred. Of these, in particular, the use of calciumoxide and calcium hydroxide permits the stable provision of ahigh-strength vulcanized rubber without changing strength propertiessuch as tensile strength according to its preparation conditions such askneading temperature. Accordingly, it is desirable to select these metalcompounds or use two or more of them in combination as necessary for theintended end application and desired properties of the resultingvulcanized rubber, and the like.

The compound of an alkaline earth metal is generally compounded in aproportion of 0.5-30 parts by weight, preferably 0.5-20 parts by weight,more preferably 1-10 parts by weight per 100 parts by weight of theethylenically unsaturated nitrile-conjugated diene copolymer rubber. Ifthis compounding proportion is too low, the tensile stress-improvingeffect is not sufficiently exhibited. On the other hand, if theproportion is too high, the strength properties show a tendency todeteriorate. Therefore, it is not preferable to use the metal compoundin such a low or high proportion. With respect to the alkaline earthmetal compound such as calcium oxide or calcium hydroxide, it ispreferable from the viewpoint of the stable provision of a high-strengthvulcanized rubber to compound it in a proportion of at least 2 parts byweight.

Examples of the organic peroxide include dicumyl peroxide, di-t-butylperoxide, t-butylcumyl peroxide, benzoyl peroxide,2,5-dimethyl-2,5-(t-butyl peroxy)hexyne-3, 2,5-dimethyl-2,5-di(benzoylperoxy)hexane, 2,5-dimethyl-2,5-mono(t-butyl peroxy)hexane andα,α'-bis(t-butyl peroxy-m-isopropyl)benzene.

These organic peroxides are used either singly or in any combinationthereof, and in a proportion of generally 0.2-10 parts by weight,preferably 0.5-8 parts by weight per 100 parts by weight of theethylenically unsaturated nitrile-conjugated diene copolymer rubber.Their optimum amount may be suitably selected according to physicalproperty values required.

In order to compound the individual components, the ethylenicallyunsaturated nitrile-conjugated diene copolymer rubber, the zinccompound, the α,β-ethylenically unsaturated carboxylic acid and thecompound of an alkaline earth metal are generally kneaded in rolls,Banbury, kneader, twin-screw extruder or the like, and the organicperoxide is then added at a temperature not decomposing the organicperoxide to knead the resultant mixture.

The resultant vulcanizable rubber composition is molded in the desiredform and then heated to vulcanize it. It is considered that the zinccompound and the α,β-ethylenically unsaturated carboxylic acid arereacted at a stage in which the vulcanizable rubber composition isprepared, to form the zinc salt of the α,β-unsaturated carboxylic acid.

Into the vulcanizable rubber composition according to the presentinvention, may be compounded various additives routinely used in rubberindustry, for example, reinforcing agents such as carbon black andsilica, fillers such as calcium carbonate and talc, crosslinking aidssuch as triallyl isocyanulate, trimethylolpropane triacrylate andm-phenylenebismaleimide, plasticizers, stabilizers, vulcanizing aids,colorants, etc.

No particular limitation is imposed on the conditions for heating andvulcanizing the vulcanizable rubber composition according to the presentinvention. However, the heating is generally performed in a range offrom 120° to 200° C. The vulcanizing process may also be suitablyselected according to the desired form, use and the like of thevulcanized rubber molding intended. It is possible to vulcanize thecomposition using, for example, press vulcanization by a hot platenpress, direct or indirect vulcanization by a vulcanizer, or a continuousvulcanizer of the drum or open type.

The vulcanized rubber molding obtained by vulcanizing the vulcanizablerubber composition according to the present invention may be used inapplications of which high tensile stress or tensile strength isrequired, for example, various seals such as packings, toothed belts,V-belts, soles for footwear, plates or sheets such as mats, continuousrubber plates, automotive fuel hoses, oil hoses, rubber rolls and rubbercoated fabrics.

The vulcanizable rubber composition according to the present invention,which develops high tensile stress, is particularly suitable for use asa material for toothed belts. In general, a toothed belt has a structurethat it comprises a main body of the belt, on which many teeth areprovided at a certain pitch in the longitudinal direction of the belt,and a tooth back in which a core is embedded in the same direction, andthe surface of the toothed part is covered with a covering canvas.Aromatic polyamide fibers, elastic urethane yarn, polyester fibers orthe like may be used in the covering canvas. The canvas is treated witha resorcinol-formaldehyde latex as needed. Glass fibers, aromaticpolyamide fibers, carbon fibers or the like may be used in the core.

The toothed belt obtained by using the vulcanizable rubber compositionaccording to the present invention in rubber layers of the tooth backand the toothed part has high tensile stress and is hence excellent inpower-transmitting ability, durability and the like. No particularlimitation is imposed on the production process of the toothed belt.However, the belt can be generally molded by inserting a coveringcanvas, a core and a vulcanizable rubber composition in order into amold and then vulcanizing the composition.

EXAMPLES

The present invention will hereinafter be described in more detail bythe following referential examples, examples and comparative examples.However, this invention is not limited to these examples only.Incidentally, all designations of "part" or "parts" and "%" as will beused in the following referential examples, examples and comparativeexamples mean part or parts by weight and % by weight unless expresslynoted.

In the following referential examples, examples and comparativeexamples, the tensile strength (TB), elongation (E_(B)) and tensilestress (100% modulus; M₁₀₀) of each vulcanized rubber sheet sample wasmeasured in accordance with JIS K-6301 to evaluate its vulcanizationproperties.

Referential Examples 1-23!

Rubber compositions composed of their corresponding formulations exceptrespective organic peroxides shown in Tables 1 and 2 were prepared by anordinary rubber kneader. Its corresponding organic peroxide shown inTables 1 and 2 was added to each of the compositions at a temperaturenot decomposing the organic peroxide to prepare a vulcanizable rubbercomposition. The thus-obtained vulcanizable rubber composition was thensubjected to press vulcanization under vulcanizing conditions of 170° C.and 20 minutes, thereby obtaining a vulcanized rubber sheet having athickness of 1 mm. The measurement results of vulcanization propertiesare shown in Tables 1 and 2.

                                      TABLE 1                                     __________________________________________________________________________           Referential Example                                                           1  2  3  4  5  6  7  8  9  10 11                                       __________________________________________________________________________    Composition (parts by weight)                                                 HNBR   100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              -- -- --                                       NIR    -- -- -- -- -- -- -- -- 100                                                                              -- --                                       NBIR   -- -- -- -- -- -- -- -- -- 100                                                                              --                                       NBR    -- -- -- -- -- -- -- -- -- -- 100                                      BR     -- -- -- -- -- -- -- -- -- -- --                                       SBR    -- -- -- -- -- -- -- -- -- -- --                                       MAA    15 20 30 48 48 48 48 48 20 20 20                                       ZnO    12 16 24 36 36 36 36 56 16 16 16                                       Mg (OH).sub.2                                                                        5  5  5  1  3  5  -- 5  5  5  5                                        MgO    -- -- -- -- -- -- 5  -- -- -- --                                       Peroxide                                                                             5  5  5  5  5  5  5  5  5  3  1.5                                      Vulcanization properties                                                      T.sub.B (Kg/cm.sup.2)                                                                398                                                                              420                                                                              412                                                                              453                                                                              444                                                                              447                                                                              427                                                                              406                                                                              387                                                                              338                                                                              322                                      E.sub.B (%)                                                                          410                                                                              370                                                                              280                                                                              150                                                                              130                                                                              110                                                                              150                                                                              110                                                                              310                                                                              240                                                                              210                                      M.sub.100 (Kg/cm.sup.2)                                                              47 82 197                                                                              378                                                                              397                                                                              408                                                                              367                                                                              392                                                                              92 154                                                                              207                                      __________________________________________________________________________

                                      TABLE 2                                     __________________________________________________________________________           Referential Example                                                           12 13 14 15 16 17 18 19 20 21 22 23                                    __________________________________________________________________________    Composition (parts by weight)                                                 HNBR   100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              -- -- -- -- -- -- --                                    NIR    -- -- -- -- -- 100                                                                              -- -- -- -- -- --                                    NBIR   -- -- -- -- -- -- 100                                                                              -- -- -- -- --                                    NBR    -- -- -- -- -- -- -- 100                                                                              -- -- -- --                                    BR     -- -- -- -- -- -- -- -- 100                                                                              100                                                                              -- --                                    SBR    -- -- -- -- -- -- -- -- -- -- 100                                                                              100                                   MAA    15 20 30 48 48 20 20 20 30 30 30 30                                    ZnO    12 16 24 36 56 16 16 16 24 24 24 24                                    Mg (OH).sub.2                                                                        -- -- -- -- -- -- -- --    5  -- 5                                     MgO    -- -- -- -- -- -- -- -- -- -- -- --                                    Peroxide                                                                             5  5  5  5  5  5  3  1.5                                                                              0.5                                                                              0.5                                                                              0.5                                                                              0.5                                   Vulcanization properties                                                      T.sub.B (Kg/cm.sup.2)                                                                512                                                                              586                                                                              557                                                                              487                                                                              453                                                                              416                                                                              393                                                                              375                                                                              288                                                                              151                                                                              290                                                                              235                                   E.sub.B (%)                                                                          520                                                                              470                                                                              420                                                                              240                                                                              210                                                                              450                                                                              310                                                                              270                                                                              170                                                                              120                                                                              150                                                                              180                                   M.sub.100 (Kg/cm.sup.2)                                                              28 48 102                                                                              235                                                                              270                                                                              74 132                                                                              176                                                                              180                                                                              124                                                                              196                                                                              126                                   __________________________________________________________________________

(Note):

(1) HNBR (hydrogenated NBR):

Zetpol 2020 produced by NIPPON ZEON CO., LTD., AN=36%, rate ofhydrogenation=90%, iodine number=28.

(2) NIR (acrylonitrile-isoprene copolymer rubber):

Polysar 833 produced by Polysar Co., AN=33%.

(3) NBIR (acrylonitrile-butadiene-isoprene copolymer rubber):

Nipol DN-1201 produced by NIPPON ZEON CO., LTD., AN=35%.

(4) NBR (acrylonitrile-butadiene copolymer rubber):

Nipol DN-002 produced by NIPPON ZEON CO., LTD., AN=53%.

(5) BR (polybutadiene rubber):

Nipol BR1220 produced by NIPPON ZEON CO., LTD.

(6) SBR (styrene-butadiene copolymer rubber):

Nipol 1502 produced by NIPPON ZEON CO., LTD.

(7) MAA (methacrylic acid):

Methacrylic acid produced by Mitsubishi Rayon Co., Ltd.

(8) ZnO:

No. 1 zinc white produced by Seido Chemical Industry Co., Ltd. (aclassified product obtained by reducing the content of coarse particleshaving a particle diameter of at least 20 μm to at most 5 wt. %).

(9) Peroxide:

Peroxymon F-40 produced by Nippon Oil & Fats Co., Ltd.; α,α'-bis(t-butylperoxy-m-isopropyl)benzene (40% of purity).

In Referential Examples 1-3, 6 and 8, high tensile stress is achievedwithin wide compositional ranges of MAA and ZnO combined with magnesiumhydroxide. More specifically, when comparing Referential Example 1 withReferential Example 12, Referential Example 2 with Referential Example13, Referential Example 3 with Referential Example 14, ReferentialExample 6 with Referential Example 15, and Referential Example 8 withReferential Example 16, it is understood that tensile stress (M₁₀₀) ismarkedly improved in the examples in which 5 parts by weight ofmagnesium hydroxide were used in combination.

As demonstrated by Referential Example 4, high tensile stress isdeveloped even when the compounding proportion of magnesium hydroxide is1 part by weight. As demonstrated by Referential Example 7, high tensilestress is similarly achieved even when magnesium oxide is used in placeof magnesium hydroxide.

As demonstrated by Referential Examples 9-11, high tensile stress isachieved even in other ethylenically unsaturated nitrile-conjugateddiene copolymer rubbers in addition to HNBR. This is apparent from theircomparisons with respective Referential Examples 17-19.

On the other hand, when magnesium hydroxide is used in combination inconjugated diene copolymer rubbers having no units derived from anyethylenically unsaturated nitrile as described in Referential Examples20-23, tensile stress is rather deteriorated.

Examples 1-12, Comparative Examples 1-9!

Rubber compositions composed of their corresponding formulations exceptrespective organic peroxides shown in Tables 3 and 4 were prepared bykneading the respective components in a kneader. In this time, thekneading was performed with the temperature of the upper limit upon thekneading varied as shown in Tables 3 and 4. The temperature of the upperlimit upon the kneading means a temperature of rubber when dumping uponthe kneading by the kneader.

Its corresponding organic peroxide shown in Tables 3 and 4 was thenadded to each of the compositions at a temperature not decomposing theorganic peroxide to prepare a vulcanizable rubber composition. Thethus-obtained vulcanizable rubber composition was then subjected topress vulcanization under vulcanization conditions of 170° C. and 20minutes, thereby obtaining a vulcanized rubber sheet having a thicknessof 1 mm. The measurement results of vulcanization properties are shownin Tables 3 and 4.

                                      TABLE 3                                     __________________________________________________________________________           Example                                                                       1  2  3  4  5  6  7  8  9  10 11 12                                    __________________________________________________________________________    Composition (parts by weight)                                                 HNBR   100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              -- -- --                                    NIR    -- -- -- -- -- -- -- -- -- 100                                                                              -- --                                    NBIR   -- -- -- -- -- -- -- -- -- -- 100                                                                              --                                    NBR    -- -- -- -- -- -- -- -- -- -- -- 100                                   BR     -- -- -- -- -- -- -- -- -- -- -- --                                    SBR    -- -- -- -- -- -- -- -- -- -- -- --                                    MAA    15 20 20 20 20 20 30 30 35 20 20 20                                    ZnO    12 16 16 16 16 16 24 24 28 16 16 16                                    Ca (OH).sub.2                                                                        5  3  5  -- 15 5  5  5  5  5  5  5                                     CaO    -- -- -- 5  -- -- -- -- -- -- -- --                                    Peroxide                                                                             5  5  5  5  5  5  5  5  5  5  3  1.5                                   Upper limit                                                                          140                                                                              138                                                                              142                                                                              140                                                                              136                                                                              162                                                                              143                                                                              163                                                                              142                                                                              145                                                                              142                                                                              143                                   temp. upon                                                                    kneading (°C.)                                                         Vulcanization properties                                                      T.sub.B (Kg/cm.sup.2)                                                                508                                                                              565                                                                              569                                                                              573                                                                              548                                                                              578                                                                              563                                                                              552                                                                              532                                                                              407                                                                              378                                                                              382                                   E.sub.B (%)                                                                          480                                                                              430                                                                              440                                                                              460                                                                              410                                                                              450                                                                              430                                                                              430                                                                              360                                                                              440                                                                              290                                                                              280                                   M.sub.100 (Kg/cm.sup.2)                                                              24 46 41 37 37 40 94 88 135                                                                              67 124                                                                              173                                   __________________________________________________________________________

                                      TABLE 4                                     __________________________________________________________________________             Comparative Example                                                           1  2  3  4  5  6  7  8  9                                            __________________________________________________________________________    Composition (parts by weight)                                                 HNBR     100                                                                              100                                                                              100                                                                              100                                                                              100                                                                              -- -- -- --                                           NIR      -- -- -- -- -- -- -- -- --                                           NBIR     -- -- -- -- -- -- -- -- --                                           NBR      -- -- -- -- -- -- -- -- --                                           BR       -- -- -- -- -- 100                                                                              -- 100                                                                              --                                           SBR      -- -- -- -- -- -- 100                                                                              -- 100                                          MAA      20 20 30 30 20 30 30 30 30                                           ZnO      16 16 24 24 16 24 24 24 24                                           Ca (OH).sub.2                                                                          -- -- -- -- 0.1                                                                              5  5  -- --                                           CaO      -- -- -- -- -- -- -- -- --                                           Peroxide 5  5  5  5  5  0.5                                                                              0.5                                                                              0.5                                                                              0.5                                          Upper limit temp.                                                                      143                                                                              164                                                                              142                                                                              161                                                                              141                                                                              142                                                                              145                                                                              142                                                                              145                                          upon kneading (°C.)                                                    Vulcanization properties                                                      T.sub.B (Kg/cm.sup.2)                                                                  453                                                                              586                                                                              459                                                                              557                                                                              478                                                                              240                                                                              250                                                                              288                                                                              290                                          E.sub.B (%)                                                                            390                                                                              470                                                                              390                                                                              420                                                                              410                                                                              190                                                                              160                                                                              170                                                                              150                                          M.sub.100 (Kg/cm.sup.2)                                                                76 48 121                                                                              102                                                                              74 138                                                                              137                                                                              180                                                                              196                                          __________________________________________________________________________

(Note):

As the respective rubber components, MAA, ZnO and Peroxide, the samesubstances as those shown in Tables 1 and 2 were used.

In Examples 1, 3, 7 and 9, it is understood that high tensile strength(T_(B)) is achieved within wide compositional ranges of MAA and ZnO.When comparing Comparative Examples 1 and 2 with each other, it isunderstood that the tensile strength of the resultant vulcanized rubbergreatly varies according to changes in the kneading temperature. If thekneading temperature is high, high tensile strength is achieved.However, if the kneading temperature is low, the tensile strength doesnot become very high. On the other hand, the comparison between Examples3 and 6 reveals that the vulcanizable rubber compositions containingcalcium hydroxide according to the present invention have no markeddifference in tensile strength even when their kneading temperaturesvary widely. The same tendencies are clearly recognized in comparisonsbetween Examples 7 and 8, and between Comparative Examples 3 and 4, inwhich the compounding proportion of MAA to ZnO was changed.

With respect to the compounding proportion of calcium hydroxide, itseffect is recognized in a wide range from 3 parts by weight in Example 2to 15 parts by weight in Example 5. However, if the compoundingproportion of calcium hydroxide is too low-like Comparative Example 5,no significant effect is achieved. As demonstrated by Example 4, hightensile strength can be similarly achieved at a relatively low kneadingtemperature even when calcium oxide is used in place of calciumhydroxide.

As demonstrated by Examples 10-12, high tensile strength is achievedeven in other ethylenically unsaturated nitrile-conjugated dienecopolymer rubbers in addition to HNBR. On the other hand, whenseparately using BR and SBR as a rubber as described in ComparativeExamples 6-7, high tensile strength cannot be achieved though elongationis somewhat improved even if calcium hydroxide is added.

INDUSTRIAL APPLICABILITY

The vulcanizable rubber compositions according to the present inventioncan provide vulcanized rubbers having excellent heat resistance and oilresistance and moreover possessing high tensile strength. Accordingly,the vulcanizable rubber compositions according to the present inventioncan be used as rubber materials in fields of which high tensile strengthis required together with heat resistance and oil resistance, forexample, wide fields of toothed belts, seals, hoses, rolls and the like.

We claim:
 1. A vulcanizable rubber composition obtained by compounding,per 100 parts by weight of an ethylenically unsaturatednitrile-conjugated diene copolymer rubber, the following components:(a)5-80 parts by weight of an inorganic zinc compound; (b) 5-100 parts byweight of an α, β -ethylenically unsaturated mono- or dicarboxylic acid;(c) 0.5-30 parts by weight of at least one calcium compound selectedfrom the group consisting of calcium hydroxide and calcium oxide; and(d) 0.2-10 parts by weight of an organic peroxide.
 2. A vulcanizablerubber composition according to claim 1, wherein the ethylenicallyunsaturated nitrile-conjugated diene copolymer rubber is a hydrogenatedcopolymer obtained by hydrogenating the conjugated diene units of acopolymer rubber of an ethylenically unsaturated nitrile with aconjugated diene.
 3. A vulcanizable rubber composition according toclaim 2, wherein the hydrogenated copolymer is a hydrogenated product ofacrylonitrile-butadiene copolymer rubber.
 4. The vulcanizable rubbercomposition according to claim 1, which is obtained by compounding, per100 parts by weight of the ethylenically unsaturated nitrile-conjugateddiene copolymer rubber, the following components:(a) 10-60 parts byweight of the inorganic zinc compound; (b) 20-60 parts by weight of anα,β-ethylenically unsaturated mono- or dicarboxylic acid; (c) 0.5-20parts by weight of the calcium compound; and (d) 0.2-10 parts by weightof an organic peroxide.
 5. The vulcanizable rubber composition accordingto claim 1, which is suitable for use as a rubber material for a toothedbelt.